An address book on a device such as a wireless device usually consists of fields such as names, home address, office address, email address and public user identities such as telephone numbers, session initiation protocol uniform resource identifier (SIP URI), mobile systems international subscriber identity number (MSISDN), among others.
When a SIP user agent registers with a network, it is supposed to provide a global unique contact address. However, in practice this is nearly never achieved and as such, the functionality of a globally routable user agent URI (GRUU) was developed and is currently in an internet engineering task force draft that is going to a request for consultation.
GRUU provides a mechanism whereby a SIP user agent (UA) can provide a globally unique identifier. This is achieved by the UA providing its address of record that is appended by an instance identifier.
The instance identifier is an identifier that uniquely identifies the SIP UA and can, for example, be a medium access control (MAC) address, international mobile equipment identity (IMEI) among others. This allows SIP UAs to set up sessions to an address of record (AOR) at a specific device.
The concept of unique identification can also be achieved in non-SIP environments. For example, a user ID such as an email address coupled with a device ID can be used to determine the uniqueness of a particular device.